1. Field of the Invention
The invention relates generally to memory devices, and particularly to systems and methods of isolating access transistor constructions for DRAM devices.
2. Description of the Related Art
Access transistors, such as Field Effect Transistors (FET's) are used in memory structures such as dynamic random access memories (“DRAMs”) for controlling access to capacitors used to store charge representing information contained in the memories. The access transistors need to be able to provide high impedance when they are turned OFF and a low impedance connection when they are turned ON.
DRAMs and other memories use an addressing scheme whereby a wordline that is coupled to many transistor gates is selected, and at the same time, a bitline or digit line that is coupled to many transistor drains is selected. An access transistor that is located at the intersection of the selected wordline and the selected digit line is turned ON, and that memory cell is accessed.
In DRAMs, charge leakage effects necessitate periodic refreshing of the information stored in the memory. In turn, refreshing of the DRAM leads to increased power consumption and delays in memory operation. Accordingly, it is desirable to reduce charge leakage effects in DRAMs.
One source of charge leakage is parasitic conductance. At the same time the access transistor that is located at the intersection of the selected wordline and the selected digit line is turned ON, many other access transistors have a drain voltage due to the drains of the access transistors being coupled to the selected digit line. These access transistors exhibit some parasitic conductance as a result of the drain voltage.
Additionally, it is desirable to minimize the area required for memories, such as DRAMs. The need for increasingly smaller semiconductors results in adjacent transistors placed closer together on the semiconductor wafer. This, in turn, results in the depletion regions of the transistors placed closer together, while still requiring electrical isolation of various circuit elements from one another. One method to create smaller depletion regions around transistors is to increase the substrate doping concentrations. However, higher doping levels increase the contaminant level in the silicon, which in turn, increases the leakage current of the transistor.
In another method to maintain electrical isolation of various circuit elements from each other, electrical isolation structures are fabricated in the semiconductors. However, electrical isolation structures require space on the DRAM or other integrated circuitry. Various techniques have been developed to reduce the area used for electrical isolation structures. As illustrated in FIG. 1, one technique for providing electrical isolation while requiring relatively little space is to place an isolation trench 102 between a portions of transistor constructions 100. However, in some types of integrated circuits, a portion of the parasitic conductance is due to corner effects that are an artifact of using trench isolation techniques.
FIG. 1 also illustrates the depletion regions 104 surrounding the gate construction of each transistor construction 100. The isolation trench 102 does not have a surrounding depletion region.